Experimental comparison of Jψ production by π±, K±, p and p̄ beams at 39.5 GeV/c

Measurements have been made of relative production cross sections of the $\text{J}\text{ψ}$ by π^±, K^±, p and p? at 39.5 GeV/c incident on copper. $\text{J}\text{ψ}$ production rates from π^?, K^? and p? are similar. The $\text{J}\text{ψ}$ relative particle/anti-particle production cross sections for x>0 are $\text{σ(π}{}^{\mathrm{+}}\text{)}\text{σ(π}{}^{\mathrm{?}}\text{)=(0.87±0.14)}$, σ(K⁺)/σ(K^?)=(0.85±0.5) and $\text{σ(}\text{p}\text{)}\text{σ(}\text{p}\text{?}\text{)=(0.15 ±0.08)}$. The small p/p? cross section ratio disagrees with models of J/ψ production by gluon amalgamation.     

Photon splitting into two photons in a Coulomb field

We explore the possibility of using neutral weak interactions to see whether neutrinos may flip their helicity. Experiments ranging from low-energy neutrino-nucleus scattering to high-energy inclusive reactions are discussed as tests for the presence of helicity-flipping scalar, pseudoscalar and tensor interactions.     

Dynamics of a monolayer of nitrogen physisorbed on graphite

The dynamics of a crystalline monolayer of molecular nitrogen physisorbed on the basal plane of graphite, which was assumed to be rigid, has been studied by means of lattice and molecular dynamics. The study focusses mainly on the registered solid in which the centres of mass adopt the √3 × √3 R30° structure, but some results are also reported for denser monlayers, including one where the centre-of-mass lattice is non-triangular. Harmonic dispersion curves are reported for two models of the N₂-N₂ interaction combined with Steele's representation of the molecule-surface interaction. The one-phonon density of states is given for one of the models. Molecular dynamics results for the same model at two temperatures, 5 and 17 K, are analysed in terms of the dynamical structure factor, and one-phonon approximation to it, and the out-of-plane motions are treated in a manner similar to the one-phonon approximation. In-plane and out-of-plane motions are largely decoupled in all cases studied, and the in-plane motions show evidence of marked anharmonicity. The out-of-plane motions appear to exhibit Fermi-Pasta-Ulam type recurrences, although this is not analysed in detail.     

A symmetry problem in polymer calculations

It is shown by a symmetry argument that the band gap in a symmetric polyene must be zero in the restricted single determinant approximation. Spurious band gaps can arise in SCF calculations when the symmetry of the lattice is not reflected in the lattice sums. The consequences of this symmetry problem are discussed.     

Event excess in the MiniBooNE search for ¯νμ→¯νe oscillations

The MiniBooNE experiment at Fermilab reports results from a search for ˉν_{μ}→ˉν_{e} oscillations, using a data sample corresponding to 5.66×102? protons on target. An excess of 20.9±14.0 events is observed in the energy range 475相似文献   

Thrombin allostery

Thrombin is a Na(+)-activated, allosteric serine protease that plays multiple functional roles in blood pathophysiology. Binding of Na(+) is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme. This review summarizes our current understanding of the molecular basis of thrombin allostery with special emphasis on the kinetic aspects of Na(+) activation. The molecular mechanism of thrombin allostery is a remarkable example of long-range communication that offers a paradigm for many other biological systems.     

Reactions within fluorobenzene-ammonia heterocluster ions: experiment and theory

Reactions occurring within gas phase fluorobenenze-ammonia heterocluster cations (FC(6)H(5)-(NH(3))(n=1-4)) have been studied through the use of a triple quadrupole mass spectrometer as well as employing density functional theory (DFT). Collision induced dissociation (CID) experiments were conducted in which mass selected cluster ions are accelerated into a cell containing argon gas and the resulting products then subsequently mass analyzed. Two dominate reaction channels are observed. The first is simple evaporative loss of neutral ammonia from the cluster ion. The second involves a substitution reaction occurring within the cluster ion to form the aniline cation, C(6)H(5)NH(2)(+), where the reactivity was found to vary as a function of cluster size. DFT calculations have been performed to both help analyze the structure and the reactivity of these cluster ions. Pronounced differences in activation energies were found that provide an explanation for the observed variation of reactivity as a function of cluster size. An ad hoc model based upon the Arrhenius equation was developed to fit both the experimental collision energy dependence of the reaction and the observed lowering of the reaction barrier to aniline formation as a function of cluster size.     

Electron impact ionization efficiency curves of van der Waals clusters

The study of van der Waals clusters is an area of growing interest and is being widely studied for a number of reasons. The measurement of the ionization efficiency (IE) curves have yielded a wealth of information by enabling ionization and appearance energies of ions to be determined which are essential for the calculation of thermochemical data. In the case of van der Waals clusters, the measurement ofIE curves enables one to determine the qualitative trends in the ionization potentials as a function of cluster size. In additionIE curves have also offered valuable insight into ionization related processes occurring in clusters. This paper will cover some of the more recent studies of Penning ionization, exciton induced decay and Coulomb explosion in van der Waals clusters through the use of electron impactIE curves.     

Single-walled carbon nanotube purification, pelletization, and surfactant-assisted dispersion: a combined TEM and resonant micro-raman spectroscopy study

Resonant Raman spectroscopy and transmission electron microscopy were used to characterize the structural changes of three single-walled carbon nanotube samples processed with purification, pelletization, and surfactant-assisted dispersion. A two-stage purification process selectively removes metallic tubes as well as small-diameter ones, enriching large-diameter semiconducting tubes. Pelletizing reduces the intertube distance but greatly increases the intensity ratio of the D band to the G band. Single-walled nanotube (SWNT) bundle size decreases during ultrasonication dispersion aided by a surfactant. SWNT bundles composed of large-diameter tubes are prone to debundling.